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FEMTOSECOND MULTIPASS
TI:SAPPHIRE AMPLIFIER
Wedge 50
User’s Manual
TABLE OF CONTENTS
1. Introduction
2. Principles of operation
3. Laser safety
4. Optical alignment
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1. INTRODUCTION
This manual describes installation and operation of Wedge 50
titanium : sapphire (Ti : sapphire) amplifier system. The system
is based on a femtosecond confocal multipass amplifier
configuration and consists of parts, shown in the Figure 1:
1. Pulse stretcher
2. Multipass Ti : sapphire amplifier
3. Pulse compressor
4. Pulse picker and Pockels cell driver
5. Synchronization electronics
The seed pulses used in the system originate from a modelocked Ti: sapphire seed oscillator, model Trestles 50 is
recommended. Before amplification, femtosecond pulses are
stretched in time to avoid effects of peak power damage in high
energy ultrafast amplifiers. Femtosecond pulses with pulse
duration 100 fs are stretched to several tens of picoseconds
before pulse selection and amplification.
To decrease pulse repetition rate, a Pockels cell is placed
between crossed polarizers. This pulse picker system permits
transmission of a single pulse during a <6-ns window that is
synchronized with a laser through the countdown and
synchronization unit. The countdown electronics receives 80 –
90 MHz signal from the output pulses of the Ti : sapphire
oscillator and divides this rate to 1000 Hz according to the pulse
repetition rate of Nd:YAG laser pumping the multipass
amplifier.
After the Pockels cell, the pulse is injected into a two-mirror
confocal multipass amplifier (MPA) that is effective device for
amplification of femtosecond pulses in a Ti: sapphire crystal.
After six, eight, or ten passes the seed pulse is amplified by a
factor up to 106 and leaves the amplifier through the aperture in
the output mirror. The pump radiation is focused by the lens
through the aperture in the input mirror.
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After amplification, the picosecond pulses are temporally
compressed to 50 - 100 fs pulses (depending on the input pulse
duration) by one-grating pulse compressor. At the pumping with
a frequency doubled Nd: YLF pumping laser (1000 Hz, 20 mJ/
pulse), the compressor gives 1.0 mJ pulses at 800 ± 20 nm.
Wedge 50 femtosecond amplifier system comprises:
1) Optical unit of two-mirror confocal multipass amplifier with
installed inside stretcher, pulse picker with HV Pockels cell
driver, and pulse compressor.
2) Synchronization electronics unit.
Synchronization
electronics
Femtosecond
Ti:sapphire
laser
Pulse
stretcher
Nd:YLF
laser
Faraday
isolator
Pulse picker
Multipass
Ti:sapphire
amplifier
Pulse
compressor
Figure 1. Wedge 50 femtosecond multipass amplifier system
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2. PRINCIPLES OF OPERATION
Femtosecond Pulse Stretcher
Temporal pulse stretching is required in avoiding the effects of
peak power damage in high energy ultrafast amplifiers. This
peak power damage is due to the tendency of bright beams to
self focusing (a result of non-linearity in the index of refraction),
which makes it necessary to limit the intensity present in
amplifiers. The technique of chirped pulse amplification (CPA)
gives a possibility to avoid this obstacle. The idea of CPA is to
stretch femtosecond pulse duration reducing peak power before
amplification and to compress pulse duration back to
femtoseconds after the amplification. A principal scheme of
femtosecond pulse stretcher is shown in the Figure 2. This is
purely optical device containing diffraction grating, spherical
mirror and plane mirrors. Femtosecond pulse going into pulse
stretcher has a broad bandwidth. For a 100 fs Gaussian pulse the
corresponding bandwidth is about 9 nm. A diffraction grating
sends different frequencies in different directions at different
angles of diffraction shown in the Figure 2 for long wavelength
(shown as red) and short wavelength (shown as blue) spectral
components of femtosecond pulse. After double pass, bluer and
redder components exit the stretcher as shown in the Figure 2.
One can see from the figure that bluer frequency components
have to travel further through the stretcher than the redder
frequency components. The result is that the redder frequency
components exit the stretcher first, the pulse has been stretched.
In the Wedge 50 pulse stretcher the input pulse is dispersed in
the horizontal plane. The stretched pulse is directed back to the
stretcher with help of vertical retroreflector, and four passes
through the stretcher are achieved. Four-pass configuration is
necessary to ensure that the stretched beam is spatially
reconstructed. Femtosecond pulses with pulse duration 100 fs
are stretched to more than ten picoseconds pulses before
amplification. High reflective gold coated holographic grating
gives stretcher efficiency higher than 50% for specific
wavelength regimes.
5
IN
OUT
VR1
SM
G1
PM
Figure 2. Femtosecond pulse stretcher
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Femtosecond Pulse Compressor
The pulse compressor was designed for compression of
picosecond pulses amplified by Wedge 50 multipass Ti: sapphire
amplifier to pulses as short as 50-100 fs. The principle of pulse
compressor operation is shown in the Figure 3. One can see that
in contrast to the pulse stretcher, redder frequency components
have to travel further through the compressor than the bluer
frequency components. The result is that the pulse has been
compressed. Varying distance between the gratings, the
compression can compensate the stretching precisely giving
almost the same pulse duration as obtained from the seed laser
pulse. The Figure 3 shows a simplified pulse compressor. In the
Wedge 50 pulse compressor some other optics are involved, i.e.
horizontal and vertical retroreflectors give a possibility to use
one grating and to achieve four-pass configuration. High
reflective gold coated holographic grating gives stretcher
efficiency higher than 50% for specific wavelength regimes.
Femtosecond Confocal Multipass Ti:sapphire Amplifier
The confocal multipass amplifier has been designed as an
effective device for amplification of femtosecond pulses in
different active media. Our unique design features two
confocally placed concave mirrors of different radii of curvature
(ROC) with central holes (Figure 4).
This telescopic
configuration provides six, eight, ten passes of the light beam
through the common focus where Ti : sapphire crystal is
placed. Due to different focal lengths of the mirrors, beam cross
section is decreased after each pass and the beam waist diameter
is increased accordingly. On the sixth pass the beam waist
diameter is about four times more than on the first one. This is
an important condition for getting the maximum gain, and one
cannot find this feature in other multipass or regenerative
amplifier systems. The pump radiation is focused by the lens
through an aperture in the input mirror.
7
IN
OUT
G2
VR2
HR1
Figure 3. Femtosecond pulse compressor
8
pump
optic axis
output
input
M1
Ti:sapphire crystall
M2
Figure 4. Optical schematic of two-mirror confocal multipass
Ti : sapphire amplifier
9
Pulse Picker
A pulse picker installed in the Wedge 50 femtosecond amplifier
system is used for one pulse selection from a train of stretched
pulses. As a result, seed pulses are formed for the amplification.
The pulse picker utilizes well known electrooptical Pockels
effect. Pulse train having horizontal polarization goes through
the Pockels cell. Without applied voltage pulses do not change
polarization and exit pulse picker with help of polarizers as
shown in the Figure 5. When half wave voltage is applied to the
Pockels cell, an input pulse changes its polarization from
horizontal to vertical, goes through polarizer and is used as a
seed pulse for the amplifier. Applied voltage is synchronized
with femtosecond pulse train and Nd:YLF pump pulses, and
seed pulses have pulse repetition rate equal to the repetition rate
of Nd:YLF pump pulses. Input polarizer is used to increase
polarization ratio for input pulses.
Synchronization Electronics
The unit is designed to trig high voltage window applied to the
Pockels cell and to synchronize this window with pump pulse
and femtosecond pulse train. The aim of synchronization is to
select one femtosecond pulse from train and to amplify it at the
maximum pump efficiency. The schematic diagram is shown in
the Figure 5 and the unit description is in the Section 5.
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Nd:YLF Power Supply
Synch. out
Q-switch
Synchronization
electronics unit
50 Hz
HV Driver
Photodiode (to trigger)
Pockels Cell
Optic Beam
Pulse picker
from Ti:Sapphire oscillator
Figure 5. Pulse picker and synchronization electronics.
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4. LASER SAFETY
Please read next page carefully before you start an
installation
WARNING ! LASER SAFETY
Be very careful executing any step of alignment. Avoid
any exposure to the directed and reflected laser beams.
Direct and reflected laser radiation from pump lasers and Ti:
sapphire amplifier can cause serious eye damage. Remember,
that Ti: sapphire radiation is invisible or looks like as red
radiation of small intensity. However, it is dangerous even at
lowest intensity. Even diffuse reflections are hazardous. Check
all reflections during alignment procedure and provide
enclosures for beam paths whenever possible. Intense incoherent
luminescence is emitted from the Ti: sapphire rod also.
Many reflections are from pump laser. Please use safety
instructions of your pump laser and use their recommendations
in your work.
Wear protective goggles whenever possible.
Keep all beams below eye level always. Never look in the
plane of propagation of the beams.
When possible, maintain a high ambient light level in the laser
operation area.
Provide enclosures for beam paths whenever possible.
Establish a controlled access area for laser operation.
Post prominent warning signs near the laser operation area:
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DANGER
VISIBLE AND /OR INVISIBLE LASER RADIATION
AVOID EYE OR SKIN EXPOSURE TO DIRECT
OR SCATTERED RADIATION
CLASS IV LASER PRODUCT
13
4. OPTICAL ALIGNMENT
Figure 6 shows optical layout of the Wedge 50 amplifier system.
Below you will find legends for all optical elements shown in the
figure. These conventional signs will be used through the
Chapter:
TIS – Ti : sapphire crystal
AM1 - spherical amplifier mirror
AM2 - spherical amplifier mirror
G1 - G2 - stretcher and compressor gratings, respectively
L1 - lens
SM - stretcher spherical mirror
PM - stretcher plane mirror
VR1- stretcher vertical retroreflector
VR2- compressor vertical retroreflector
HR1- compressor horizontal retroreflector
P1, P2, P0 - polarizers
PC - Pockels cell
M1 – M9 - mirrors, high reflectors at 800 nm
Y1 – Y3 – mirrors, high reflectors at 532 nm
A1 – A5 - apertures
BS1 – BS2 – beamsplitters
R – half wave plate
F – Faraday rotator
INPUT BEAM ALIGNMENT
1. Remove the rotation stages with gratings from the breadboard.
2. Operate the Ti : sapphire seed oscillator in continuous wave
regime at 800 nm.
3. Input beam should be aligned through apertures A1, A2 and
A3 at a beam height of 120 mm above the breadboard.
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15
Y2
M9
L1
M8
IN
BS1
AM2
SM
M1
IN
OUT
M5
A1
VR1
A4
P2
PC
TIS
G1
R
P1
F
G2
P0
Y3
A2
PM
A5
VR2
M2
AM1
IN
M7
M6
to pin-diode
OUT
BS2
A3
HR1
Figure 6. Schematic layout of multipass amplifier, pulse stretcher, pulse picker, and pulse compressor
SEED
PUMP
Y1 to trigger
M3,M4
OUT
STRETCHER GRATING ALIGNMENT.
1. Replace the rotation stage with the grating G1. Set position of
rotation stage to “0”.
2. With rotation stage set so that the zero order beam from the
stretcher grating goes back through aperture A1, adjust the
vertical and horizontal tilts of the grating mount for best
alignment to aperture A1.
3. Set rotation stage at 14 degrees for 800 nm. Littrow (auto
collimated) diffracted beam should go back through A1.
4. If not, adjust rotation of the grating about it‟s face for best
vertical alignment to the A1. These is done by carefully
loosening the retaining screw in the grating mount so that the
grating holder can be rotated in the mount . Caution: grating
holder can fall from tilt stage if this step is not performed
carefully or if retaining screw is too loose..
5. Reiterate between step 2 (adjust only the vertical tilt of the
mount) and step 3 (adjust only the rotation of the grating) until
no further adjustment is necessary.
STRETCHER MIRROR ALIGNMENT
1. Operate the Ti : sapphire laser in continuous wave regime at
800 nm.
2. Rotate stretcher grating angle so that first order diffracted
beam hits the center of mirror SM. The beam height must be
maintained at 120 mm. The distance from SM to the center of
stretcher grating G1 should be 35 cm. The angle between input
and diffracted beams is equal 14 degrees (rotation stage position
should be approximately at 21 degrees).
3. Adjust mirror SM, so that reflected beam hits the center of
mirror PM. The distance from the mirror SM to the mirror PM
should be 70 cm.
4. Adjust mirror PM, so that reflected beam hits the mirror SM
at the height of approximately 114 mm. The second reflected
from SM beam will hit the stretcher grating at the height of 114
mm. The second reflected from G1 beam hits the lower mirror
of the stretcher vertical retroreflector VR1. Exiting
retroreflector, the beam hits the G1 at the height of 126 mm and
reflected beam hits the SM at the height of 126 mm, too. The
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third reflected from the SM beam hits the PM. The reflected
from the PM beam hits the SM at the height of 108 cm. The
fourth reflected from the SM beam hits the G1 at the height of
108 cm, too. The fourth reflected from the G1 beam hits the
mirror M1. Radiation pattern on the stretcher grating G1 is
shown in the Figure 7:
Figure 7. CW radiation pattern on the stretcher grating
If the spot No 3 is not aligned vertically with one another, a
small horizontal rotation of the VR1 should be done. When the
Ti:Sa laser is mode-locked, pattern seen on the G1 is shown in
the Figure 8:
Figure 8. Femtosecond radiation pattern on the stretcher grating
PLANE MIRROR (PM) ALIGNMENT
You must check a view of the reflected from M1 beam on a
distance about 3 m. If you moved the PM translation stage, the
shape of the beam is changed. You must find the position of the
PM when the beam spot is round. Two spots observed on the
PM should coincide in case of perfect stretcher alignment. The
stretcher alignment is finished with this procedure.
FARADAY ASSEMBLY ALIGNMENT
1. Direct the beam through R, PO. Do the clamp of R more
weaker. Rotating R measure the average power behind PO
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(PO has been installed at 45o position). Get maximum of the
average power. Press the clamp of R.
2. Direct the through F with help of M2.
PULSE PICKER ALIGNMENT
1. Remove Pockels cell PC and polarizers P1 and P2.
2. Adjusting M1, direct beam reflected by M1 to the center of
M2.
3. Adjusting M2, direct beam reflected by M2 to the center of
M3 and M4. Mirrors M3 and M4 serve for 90o polarization
rotation.
4. Replace P1 and adjust it. The input beam should hit the center
of P1. P1 should be oriented for transmission of light having
horizontal polarization.
5. Replace P2 and adjust it. The input beam should hit the center
of P2.
6. Place white screen (white paper sheet) after P2 and observe
transmitted by P2 light with IR viewer. Mark spot position by a
pencil.
7. Rotating P2 find minimum transmutation.
8. Replace PC and adjust it. The input beam should hit the center
of PC.
9. Place a sheet of scattering paper (for example, a sheet of
optical cleaning paper) between P1 and PC.
10. With help of IR viewer observe on the screen a spot of the
transmitted light. The spot looks like dark cross. Adjusting PC,
coincide the center of the cross with the mark on the screen.
11. Remove the scattering paper.
12. Achieve minimum light transmutation through P2 by fine
adjustment of PC.
AMPLIFIER ALIGNMENT
1. Place a half wave plate between P1 and PC or between PC
and P2.
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2. With help of M4 and M5 direct the beam into apertures A4,
A5. Remove A4, A5. In case of aligned amplifier go to step 16.
Go to step 4 if it is necessary to make amplifier alignment.
3. With help of M3 and M4 direct beam onto left side of M5.
4. With help of M4 and M5 direct beam parallel to the
breadboard at the level 132 mm above the breadboard.
5. Place amplifier mirror AM1 (R=1000 mm) and AM2 (R=830
mm) at the distance 915 mm between metal clamps. The
distance between AM2 and M5 should be 10 cm.
6. Set AM1 hole centers at the same level above the breadboard
as the beam level. AM2 hole center is at 142 mm.
7. With help of M5 direct the beam onto AM1 to a distance 18
mm from the edge of the AM1 hole to the center of the spot.
8. With help of AM1 direct reflected beam to the right side of
AM2 to a distance 17 - 18 mm from the symmetry center of the
AM2 to the center of the spot.
9. With help of AM2 direct reflected from AM2 beam parallel to
the M5-AM1 beam (47 mm distance between beams). After this
step, multipass beam configuration will be aligned automatically.
Six-, eight-, or ten-pass amplifier configuration is determined by
the distance from the center of the M5-AM1 beam spot to the
edge of the AM1 hole.
10. With IR viewer watch six (eight) beam spots on mirrors
AM1 and AM2 (Figure 9).
Figure 9. Radiation pattern on mirrors AM1 and AM2
19
11. Moving narrow paper strip through beams between AM1
and AM2 find common for AM1 and AM2 focal point.
12. With horizontal control of AM2 tune the beams
configuration to get 40-41 cm distance between AM2 and
common focal point.
13. Place the titanium : sapphire crystal assembly to the common
for AM1 and AM2 focal point. A movement of the lowest
translation stage should be parallel to the M5-AM1 beam.
14. Aligned multipass configuration will be broken after the step.
By horizontal control of M5 take all beams to one spot on the
left surface of the titanium : sapphire crystal. After that, restore
eight passes configuration slightly moving AM1 translation
stage in a direction “to the user”, and simultaneously adjusting
horizontal control of AM1.
15. Using controls of Ti:sapphire assembly and controls of AM1
and AM2 get that:
a) Going to the crystal input beams should intersect in one and
the same point on the crystal surface (use microscope).
b) Going through the AM1 hole output beam should not touch
the edge of the AM1 hole.
16. With help of M6 and M7 direct the output beam to the pulse
compressor.
17. Place substrate beamsplitter anywhere between M6 and M7
and direct reflected beam to pin-photodiode connected with
oscilloscope for amplification control. Watch unamplified
pulses.
COMPRESSOR GRATING ALIGNMENT.
1. Use mirrors M6 and M7 to align the beam though the
apertures A3 and A2.
2. Replace the rotation stage with the grating G2. Set position of
rotation stage to “0”.
3. With rotation stage set so that the zero order beam from the
stretcher grating goes back through aperture A3, adjust the
vertical and horizontal tilts of the grating mount for best
alignment to aperture A3.
4. Set rotation stage at 14 degrees for 800 nm. Littrow (auto
collimated) diffracted beam should go back through A3.
20
5. If not, adjust rotation of the grating about it‟s face for best
vertical alignment to the A3 by carefully loosening the retaining
screw in the grating mount so that the grating holder can be
rotated in the mount. Caution: grating holder can fall from tilt
stage if this step is not performed carefully or if retaining
screw is too loose..
6. Reiterate between step 3 (adjust only the vertical tilt of the
mount) and step 4 (adjust only the rotation of the grating) until
no further adjustment is necessary.
COMPRESSOR ALIGNMENT
1. Use mirrors M6 and M7 to align the beam through the
apertures A3 and A2.
2. Rotate compressor grating angle so the angle between input
and diffracted beams is equal 22 degrees (rotation stage position
should be approximately at 25 degrees). The input beam entering
the compressor should be reflecting off the lower right corner of
the compressor grating G2 before entering the horizontal
retroreflector HR1, as shown in the Figure 10:
Figure 10. Femtosecond radiation pattern on the compressor
grating
3. The HR1 should be reflecting the stripe onto the lower left
side of the grating. If the side to side position is off, the lateral
position of the HR1 can be adjusted by loosening bolts holding
the HR1 assembly to the translation stage.
4. The first stripe should now reflect off the vertical
retroreflector VR2 before reflecting off the grating G2 a third
time. The third reflection on the grating should be a stripe on the
upper left side of the grating directly above the first stripe. If the
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height of this stripe requires adjustment, move VR2 in vertical
direction by loosening two bolts.
5. The beam should now reflect off the HR1 a final time before
reflecting off the upper right corner of the grating in the form of
a spot. The beam should now be exiting the amplifier optical unit
above the mirror M7 and through the output port. The height of
the final compressed beam is adjusted with HR1 assembly.
6. The length of the compressor should be set by adjusting the
horizontal retroreflector HR1 to be 30-32 cm from the grating to
the apex. This should be adjusted further by minimizing the
pulse width as measured by autocorrelator.By changing the
angle of grating and adjusting the horizontal HR1, you can
minimize the pulse width.
PULSE AMPLIFICATION
1. Place neutral filters before pin-diode to detect amplified
pulses.
2. Switch-on pump Nd:YLF pump[ laser for attenuated “free
generation” operation (see Operation Manual for the pump
laser).
2. With help of two external mirrors Y1, Y2 direct attenuated
pump beam into amplifier through the center of the AM2 hole
onto the Ti:sapphire crystal surface (to the point where amplified
beams are intersected).
3. Place lens L1 into the pump beam maintaining the direction of
the pump beam. A distance from L1 to the TIS (approximately
680 mm) is determined by an initial diameter and divergence of
the pump beam. To find this distance watch with microscope a
diameter of the pump beam on the crystal surface. It should be
about 750 .
4. Set up concave mirror Y3, reflecting pumping beam back to
the Ti:sapphire crystal. A distance between the mirror and Ti:
sapphire crystal (approximately 205 mm) should be adjusted to
give reflected pump beam waist diameter about 750 inside the
crystal.
5. Switch-on „Q-switch” operation of the pump laser. Set pump
energy Epump = 20 mJ.
22
6. Watch amplification with pin-diode and oscilloscope, slightly
moving pump beam with external mirror.
7. Get maximum amplification slightly adjusting AM1, L1, Y3,
and M5.
8. Pulse amplification is determined by amplification saturation
level. An amplification saturation is highest when only 3-4
amplified pulses are detected with oscilloscope.
9. Check energy of amplified pulses at 8-pass amplifier
configuration, Epump = 20 mJ, and 750 pump beam diameter
inside the TIS. Average power of a train containing 3-4 pulses
should be not less than 70 – 80 mW after pulse compressor
when 50 Hz pump laser is used.
10. Remove half wave plate.
11. Switch on the electronics unit.
12. With controls of the synchronization unit change a delay of
the selected femtosecond pulse and achieve maximum
amplification for the selected pulse. Average power of amplified
pulses should be 50 – 55 mW when 50 Hz pump laser is used,
and spontaneous emission should not be more than 8 – 10 mW
at closed input for seed femtosecond pulses.
23